1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive using a dual slope integrator to extract a velocity of an actuator arm from a back EMF voltage.
2. Description of the Prior Art
Disk drives employ embedded servo sectors to derive the velocity of an actuator arm during normal seeking and tracking operations in order to control the speed of a voice coil motor (VCM). However, there are times when the embedded servo information is unavailable, such as ramp loading/unloading, when it is still desirable to derive the velocity of the actuator arm in order to control its speed in a closed loop servo system. The prior art has suggested to derive the velocity of the actuator arm by processing the back EMF voltage present across the voice coil of the VCM. The VCM is essentially an RLC circuit where R is resistance, L inductance, and C the inertia of the motor and load. The voltage contribution of C to the measured back EMF is proportional to the velocity of the actuator arm. By canceling the contribution of R as well as Lxc2x7di/dt, the remaining back EMF voltage represents the velocity of the actuator arm.
Accurately canceling the contribution of R from the back EMF voltage requires compensating for the fluctuations in R, for example, fluctuations due to changes in the ambient temperature as well as self-heating. Accurately canceling the contribution of Lxc2x7di/dt requires an accurate estimate of the VCM""s transient response. The prior art techniques for calibrating the VCM""s resistance R typically include a calibration procedure executed periodically, such as pressing the actuator arm against a fixed object to measure the back EMF voltage contribution of R. However, the accuracy of the estimated R depends on the frequency of the calibration procedure which can degrade the performance of the disk drive. U.S. Pat. No. 6,097,564 discloses a method for estimating the VCM""s transient response using a sophisticated xe2x80x9cmodel simulatorxe2x80x9d in order to cancel the contribution of Lxc2x7di/dt from the back EMF voltage. However, this solution is complex, expensive, and error prone since the model simulator only approximates the VCM/servo-driver transient response.
There is, therefore, a need to accurately and cost effectively cancel the voltage contribution of the VCM""s resistance R and the contribution of Lxc2x7di/dt from the back EMF voltage to derive an accurate velocity estimate for the actuator arm when embedded servo information is unavailable.
The present invention may be regarded as a disk drive comprising a disk having a plurality of tracks, an actuator arm, a head connected to a distal end of the actuator arm, a voice coil motor comprising a voice coil, the voice coil motor for rotating the actuator arm about a pivot to actuate the head radially over the disk, and a velocity extraction circuit for extracting a velocity of the actuator arm from a back EMF voltage across the voice coil. The velocity extraction circuit comprises an integrator. During a first time interval the integrator integrates a first voltage representing a current flowing through the voice coil to generate a first integration, and during a second time interval integrating a second voltage across the voice coil to generate a second integration. A polarity of the first voltage is opposite a polarity of the second voltage, and the velocity of the actuator arm is computed by summing the first and second integrations.
In one embodiment, the voltage extraction circuit comprises a sense resistor connected in series with the voice coil for detecting the current flowing through the voice coil, wherein the first voltage is measured across the sense resistor.
In one embodiment during a calibration mode when the velocity of the actuator arm is substantially zero at least one of the first and second time intervals is adjusted until the sum of the first and second integrations substantially equals a predetermined value. In another embodiment, the integrator comprises an amplifier and a variable resistor for setting the gain of the amplifier. During the calibration mode when the velocity of the actuator arm is substantially zero, the variable resistor is adjusted until the sum of the first and second integrations substantially equals a predetermined value.
In one embodiment during the calibration mode the actuator arm is pressed against a fixed object. In another embodiment, during the calibration mode the head is maintained over a centerline of a target track during a tracking operation so that the velocity of the actuator arm is substantially zero on average.
In one embodiment, the integrator comprises an amplifier and a capacitor.
In yet another embodiment, the velocity extraction circuit waits a third time interval following the first and second time intervals. During a fourth time interval, the integrator integrates a third voltage across the sense resistor to generate a third integration, and during a fifth time interval the integrator integrates a fourth voltage across the voice coil to generate a fourth integration. A polarity of the third voltage is opposite a polarity of the first voltage, and a polarity of the fourth voltage is opposite a polarity of the second voltage. The velocity of the actuator arm is derived relative to the sum of the first, second, third and fourth integrations. In one embodiment, the fourth time interval is greater than the first time interval and the fifth time interval is greater than the second time interval. In another embodiment, during a calibration mode when the velocity of the actuator arm is substantially zero the third time interval is adjusted until the sum of the first, second, third and fourth integrations substantially equals a predetermined value.
The present invention may also be regarded as a method of extracting a velocity of an actuator arm in a disk drive from a back EMF voltage across a voice coil of a voice coil motor. A first voltage across a sense resistor in series with the voice is integrated during a first time interval to generate a first integration. A second voltage across the voice coil is integrated during a second time interval to generate a second integration. A polarity of the first voltage is opposite a polarity of the second voltage, and the velocity of the actuator arm is derived relative to the sum of the first and second integrations.